Defluorination of phosphate rock



le, 1949/ A.

time:` availability jol of workV y litige amount.' has heretofore ben t considered essenl 'the of theapatlt mtice ao :roei-the noorine. In some quarters. it was believed that the' necessary for combinationA with the ffl'ee `limestone or calcium carbc n1 a;i j.e,` ll'lwith' the oompoundsof 'ix'olrand aluminum customarily present in most phosphate rocks; In` other'equarters.,` silica was behevednec'essary ill'folx'tlcrfto l combine with the iiuorinei' "Inth' United States y Letters Patent of one of us (Maust)No;-2,4`46,978, Patented August 10.1194. 1F Wasrecosnizedifcon- "of, that appllcatlooracordlnsly.:involves1 theedwith `-izhe phosphatelxock of vvfrom to (utili-Jj l f "w'efhave discovered-that iflittlefor no silica is v byA lshtklod' silica 1 a1 oalcilling tempersilica" in` amount "p" to abollt` 30% y g'exelally promotes fusionuotu Vthe phosphate rocket tempera.-

V``'mirtloloifg*with omV 'anotheiand consequently luvsion mdstlokmgtofetner of theypmlcles lsthus l avoided. yvilefhave' now rdiscovered *that if* the silica content' of the phosphate rock is sumciently u; low; effective aenuorlnauon `los' calclnationlfthe www water mor lsf possible temeraties in" excessof 2590 i". `without ye'xloountel'iris f withoutthe necessityjof carefully controlling-the calcillingr-telperatllre which may ultimate1y`rlse Jto approximately3`000 F. Thus,

present Ain 'the phosphate rock; there isrth'eil nothly inguin thefrock to'cause *fusion* at the tempervaizux'ell necessary to' f'rex'noye thef'tluol'lne by :the 2 faction ofy watervaporfal'ld the 1 reaction between `the water. vapori and'i'iu'ori'e content-of the rock unim'peded by fusion".f`^We have 'found xthstthesilicafcontent 'ofthe rock should be less than 4%,i-fmd` "preferably less `than 13%."*Sinoe 301 `most native' phosphate @rocks i 'contain` `substantiaJly larger-percentages of *silica-pthesilica must berelnoved, atleast in part. 1

" f Theefollowingtablelglvs a typilalnlysi for severa1:phospha'te rocks" mined 'in the 'United caracas It will therefore be seen that if there is nothing (such as silica) present to combine with the oxides of calcium, iron and aluminum. fusion will not take place at deuorinating temperatures of from 2500 to approximately 3000 F., and in the absence of fusion water vapor is able to penetrate the phosphate rock, preferably nodulized, and react with the fluorine as follows:

Based on the foregoing discoveries, our present invention involves calcining, in the presence of water vapor, a charge composed predominantly of finely divided phosphate rock (or equivalent phosphatic material containing iiuorine) and containing less than 4%, and preferably less than 3%, of silica at a temperature of at least 2700 F. but insuilcient to cause any substantial fusion oi' the charge. The calcining temperature is preferably between about 2700 F. and about 3000 F., within which temperature range substantial deiluorination is effected in from to 30 minutes. It is essential that the water vapor penetrate to all parts of the charge during the entire period o deuorination.

Most phosphate rocks, as mined, contain too -high a silica content for direct treatment in accordance with the present invention. Silica may be removed to the necessary extent in any appropriate manner, as, for example, by flotation, electrostatic separation, sorting, or other silicaseparating process. In most cases. the phosphate rock must be crushed and ground to a proper fineness to liberate mechanically bound but free silica, a flneness of minus 48 mesh (standard Tyler screen) being generally adequate, although with some rocks a iineness of minus 200 mesh is necessary. Since for deuorination it is desirable that the rock be ground so that at least 65%, ,passes through a 200 mesh screen and all passes through a 65 mesh screen, it is advantageous where possible to grind the rock to such a neness in preparation for the removal ofsilica. By free silica is meant that present as pimples in the phosphate rock and plainly seen by means of either the naked eye or a microscope. The combined silica is that present in combination with other elements, and which cannot be removed by the ordinary silica-separating processes. It sell dom is present in excess of 2%, and often is under 1%.

In the phosphate industry, silica is customarily determined as "insoluble matter in one part of concentrated hydrochloric acid and one part of water at boiling temperature. The insoluble matter will consist for the most part of silica but may, and usually will, contain a relatively small amount of other insoluble substances. So far as the practice of the present invention is concerned,

these other insolublesubstances are unobjectionable and probably act in much the same manner as silica. Since in commercial practice, determinations of insoluble matter are, more usual and more readily made than determinations of silica, it is generally more convenient. in practicing the invention, to consider the insoluble matter as the equivalent of silica, and throughout this specication and the appended claims the contemplated low percentages of silica in the calcining charge are to be considered as in the form of insoluble matter.

In carrying out the invention, the phosphate 'rock should be in a finely divided state, preferably so that at least 65% passes through a 200 mesh standard Tyler screen 'and all passes 't through a 65 mesh screen. While the phosphate rock of low silica content may be calcined in its nely divided form, superior results are generallyattained by forming the finely divided rock into nodules, pellets, briquets or the like. Nodulizing or the like may be carried out as a preliminary or Cfr preparatory operation or may be effected in situ immediately preceding calcination. For example, suitable nodules, generally round in shape and varying in diameter from to 1 inch, may be made by moistening the finely divided rock with water or other suitable liquid and tumbling at room temperature in a rotating cylinder, barrel or the like. From 12 to 18% by weight of water or the like, and generally around 15%, will sufciently moisten the dry nely divided rock for producing satisfactory nodules by tumbling. Nodules may also be made by drying an aqueous slurry of the rock, and cutting the dried product into suitably sized cubes or other shapes. Nodules may also be made mechanically by briquetting or the like, although nodules so produced are gen- Verally too dense or compact for effective subsequent penetration of the water vapor. When cal,- cination is carried out in a rotary kiln, nodulizing may conveniently be effected in situ by making a slurry of the finely divided phosphate rock with from 40 to 50% by weight of water, and heating the slurry and evaporating the water4 in the low temperature end of the rotating kiln.

Calcination can be carried out in any suitable apparatus, such as a rotary kiln, electric furnace, shaft furnace, sintering machineetc. The multiple hearth shaft furnace disclosed in the copending patent application of one of us (Maust), Ser. No. 679,178, filed June 25,4 1946,-.is a particularly suitable apparatus for the practice of the invention. In whatever apparatus em.' ployed, intimate and continuous association. of the water vapor with the entire body or charge of phosphatic material undergoing calcination is necessary. As rapidly as water vapor is consumed in the operation, fresh water vapor must be available to instantly replace that consumed. and free evolution of the resulting gaseous products (e. g. iluorine 0r hydrogen fluoride, carbon dioxide, sulphur oxides etc.) is necessary. Heating in the presence of water vapor is thus corn tinued until the desired elimination of fluorineis attained. In general, the higher the temperature of calcination the shorter is the required detention period at that temperature to effect the contemplated deuorination Y The invention, in its broad aspect, is illustrated in the following examples in which the phosphate rock was a Florida pebble phosphate which had been subjected to notation to remove silica. The

analysis of the resulting low-silica rock was as The finely ground phosphate rock was nodulized in a, cylindrical rotating nodulizer by means of a spray of water. Bentonite (0.5% by weight) was added to the dry ground phosphate rock in order to give the nodules residual strength after drying as well as during calcination. The dry nodules were calcined in an electric furnace through which water vapor was continuously passed in intimate contact with the hot nodules. Twenty t Substantial fuslonforsintex'jingof the charge tn be l consideredV asl fusion, or sintering minutes were takenln gradually bringing the nodmes up to the @leitung temperatures mdp "dmthefdmmlbl???` i `V ass a temperature of about 2900* in order to obtain `substantial denomination t However', invadere.i m

`an'ceVfvviFhi theflnvvention as represented by the `foregoing' example, ,themphosphte did' not `fuse at `that calclnlngtemperature. On the other j hand; wwith the jsame Florida pebme. phosphatev having a. customary slicacontent of about-6% s. hopeless fusion'talkesy place at a. temperature of even 2700 Fggthus preventing reaction of the water vapor within theinterlor ,of a nodule, and

inaklng axsticky; massthat ls unworkablelin coxn'-,4 1: mercil rotarykuns ,brother furnaces.

`calcining `tfsuch` extentthat lthe Chargey becomes Stinky,

" j" m wholeyorwm ddltends tdfclmggdrstii; td the "waugof, the` caljemlng apparatus. Fusione may alsobef-deflnedffsla condition where for "l 'given tix'nepc'eiod,ol` treatment a` higherpuorlne contentjres'nlts than fronitreating yfor zth'efsame tmefperiod atia lower temperature, substan-l tml fusion dr"sintnngfattemperatures below that" `z required for 'effective deuuo'rinatidn impedes and sion takes piece; the'nodules contacting with'the clned for the same time at a hig` er temperature;

` ltls a sign that fusiontookh'placejwhen the nodule flrst` 'enteredjthe hot fonegof `calclnatlonthereby substantially prev ting further deuor ation illustrate the *thannnerl l "Thiel following "examples unts ` degree of porosityv tothe nodules, as more particmany described `herenmter.ff"-nentomte (05%) was alsofincluded in the 'mixture strengen th debqelemdwe;

Vying., Temo- `ses;iraient'jl is of the order of six mlnuteswyHencein the Dresr ent lnventlon,-weflnd ltadvantageousnot to n Erely upon the b eivxelcinl,el'fecytt's` ot` silica.: but

,peraturese Sffin the neighborhood of 2900? F., thus `stretching0111: the` hot 29T-les. of "2800".A F., r`2TI00F *26005K*y 94161;509"1 Ff, lWithout substantia1"lfusion `offthe jphosphatlc Vmaterial;undergoing 4h, caleination, so` that the, periodgfatvlwhlchywater vapor can effectively react with luolfineis,v ,-extended, andk `we do not encounter thefiuslon problem that prevents good kilnoperatlon. el .When ,the `'wllliv- =11ttrerns.-ndulizedfthe A 1 noduleslshould `have` a high degree ofy `porosity in .wenn Ymiddle' @dus @einem @daenemation. i Such porosity mayladvantageouslyybe lm- Eflafld t0 2th? by fthe, invention ofgfour ueopendngJgiatelity application, Ser;` N0. l 665,348, led April 26,1946 l. In accordance with that yln- A vlention, "a ',hlghly: effective" porosity imparted l tov thev nodules tbyl includingjroml Snijd-50%; l1V 5'5, wegnht :of ca.,rbonaceous material in the modul yand eliminating :substantlally all, of cthemarbt'xn of the crbonaceos material by reactiong-with. water vapor at a temperature in excess of 1800 F. f? with? evolution of the resulting gaseous products. @At that temperature,` thewater vapor reacs'with 'the hot carbon of *ther carbonargeoixs` material with thev evolution of hydrogen and 'carbonjvmonoxlde i in much the'same way that watergas'is1 forn'ed. The heat treatment* in' the presence fof Water vapor is continued until the nodules aredecar- -bonized for. all `practical' purposes;4 :that: is :until ,substantiallyfvall of '1, the carbon i has ben'f elimina.'tedf,`,ileaving* the nOduleS wllllhf4 desired=d n 1gree.ofiporosityggtfl'he l decarbohilng "temperature Vmayxadvantageously" be frorn 20'00fto 2400E F.

" duodfnighldtmperatureshdld be avoided.y {sl'nce it may i cause phosphorus y -'Vre- *d "We affthiphosnhetmeriel byticerbsmi- 0r `ma cause sucnjsubstan'ttalAzrdsxpnjorsmtenng of ',ofvcalcinatlonand be gcontactdwithwater vapor tratewthroughoutftheentire mass ofeeaehgulndicarbon is not eliminated. n the other hand, a

slight amount of sintering is advantageous since it imparts a desired amount of strength to thedecarbonized porous nodules. Thus, in the case of most finely divided phosphate rocks, incipient sintering takes place at about the same temperatures at which the water vapor reacis with the carbonaoeous material, and this slight incipient sintering is suilicient to convert the nodules into relatively hard clinlrers. Hence, when porosity is attained by the removal of carbon, the clinkered nodules are suiliciently strong to withstand subsequent handling in the calciaing kiln or furnace without disintegration.

A wide variety of carbonaceous materials are available for imparting porosity to the charge. Among these may be mentioned, by way of example, bituminous or anthracite coal, coke, charcoal, lamp black and other forms of carbon. liquid and solid petroleum products, waste sulphite liquor, ilour, distillery slops, sawdust, ground up grape fruit peelings etc. Solid w. 'm materials are crushed (when and linely ground, preferably so that at least '15% pases througha200meshstandard'1'ylerscrmand substantiallyallthroughameshscreen. At least by weight of carbonaceous material is required to impart any effective degree of porosity to the nodules, and generally at least is preferred. The higher the percentage of carbonaceous material initially included in the nodules,

the higher will be the porosity of the decarbonized nodules. In practicing the present invention with phosphate rock charges containing les than 4% of silica, the nodules may initially contain up to 50% by weight of ma terial, from to 40% by weight usually giving satisfactory results. f

When porosity is imparted to a nodulixed charge by the initial inclusion of a substantial amount of carbonaceous material and subsequent decarbonization, substantially complete deiluorination can often be eifected at a lower calcining Y Percent Total P205 40.30 Insoluble (S101) 3.77 FezO: and A1203 1.26 li'luorinel 2.52 Fineness l 69.2

l Minus 200 mesh.

The coal was a high grade bituminous coal (volatile matter 29.1% and ash 7.2%) and was crushed toaiineness 83.4% minus200mesh. 'Ihenely ground phosphate rock and coal (and 0.5% bentonite in each case) were intimately mixed together in different proportions. The mixture was madeintheicrmofa slurry, drledandcut-upinto 54 inch cubes. The cube nodules wee rst heated at a decarbonizing temperature increasing from 1800F.to2500 F.,inanelectricfnrnace sure was continuously 1wearbon was eliminated. The temperature oi the furnacewasthenraisedtothe deiluorinating temperature of 2600 F., and the nodul were held at that temperahire in the of water vapor lmder a slight-mesure for the varying indicated time intervals, the ridual ilumine contentbeingdeterminedaftereachcfthesedilerent periods of calcination. The slightwatervapcr pressurewasneededtoassuredirect of the nodule with water vapor to both remove the coal by destructive reaction with the water vapor and to remove the fluor-ine by the action of the water vapor.

Puemtnminemvarylngtilal Mimtcldnmg damm- Temp- Nocnai 21% coal sul ed 3.5 1.42 1.2 l. 7.. L5 -7l .5l .8 14 LU .ll .W .I I). .0l .a .Il

. covered.

As hereinbefore we prefer. whm ncdulizingthecalcining chargetoineludefnan 0.5to2.0% byweightofbentoniteintheelnrge mixtureinordertoimpartadesirabledegreeot hardness andtothe drhd'noduleato withstand subsequent handling,andto prevent objectionable dusting of the noduliaed charge during calclnation, especially in a. rotary kiln. Some phosphaterocks,suchfor exampleasllol'- ida pebble rock and especially the phosphate rocksfromNorthAtrica,havewheniinelygmund suilicient natural colloidal constituents to form strong nodules upon drying, and hence require but a small addition (e. g. 0.5%) of bentmlte. Ontheotherhaniphosphaterocksofthcapa titetypehavelittleornnnatlnaloolhidaleonstituents.andasmuchas2%by weightofbentonite may advantlgeouslybe inclllied in the c81- cining charge to impart the desired degree of strengthtothedriednodules.

Whileweprefertoimpartporositytoanoduiizedcaiciningchargebyofinitiallyincluded w materiaLasherdnbeforedescribemadesireddegreeofpontymy beobtainedbyincludinginthecalciningeharge somesubstancetlmtvdalizegupontheapplication ofheat,suchasammoniumca1bnte.or sulphurwhichburmd'fassulplmrdlmidgcl oneofalargevarietyofhm'htnsimmlplnt,

throughwhichwatervaporunderallightpres-carbonates. iluorldes etc. whichlnae waterot aucgeac hydration upon heating.' Po'rosity may also be obtained by mechanical means such as the addition of a frothing agent to an aqueous slurry of the charge mixture, and then vigorously agitating the slurry, whereupon the slurry dries with a iiuft'y or porous structure.

The following examples illustrate the beneiit, in practicing the invention, of imparting porosity to the nodules by a frothing or iiuiling-up treatment. In these examples, a low insoluble (2.68%) Florida pebble rock was used, having approximately the same analysis as hereinbefore recited. The finely divided rock was made into a slurry using approximately 35% water. In Example I, the slurry was dried and cut into .inch cubes. In Example II, a soya bean extract was added to the slurry, and the slurry was agitated or iiuffedup, dried and cut into 3A; inch cubes. In Example III, egg albumen was added to the slurry, which was then uffed-up, dried and cut into inch cubes. Since the iiuffed-up slurry has little strength upon drying, a hardening agent (starch or flour) was added to the slurry in Examples II and III. The porous cube nodules should be quickly heated to a temperature of 2000-2200 F., at which slight sintering takes place, in order to retain their form and porosity after the organic hardening agent burns off. In each example, the nodules were heated at a temperature of 2700 F. in the presence of water vapor, and the fluorine content determined after the indicated time in'- Percent iluorine alter calcination at 2700 F. iur- 5 minutes V 10 minutes We claim:

1. The method of defluorinating a phosphate rock containing silica in excess of 4% and` nuorine in excess of 1%, which comprises subjecting the rock to a silica-removing treatment and reducing its silica content to less than 4%, calcining without substantial fusion a charge composed predominantly of the treated rock and containing less than 4% of silica at a temperature of at least 2700 F. but not so high that substantial fusion takes place, and subjecting all parts of the charge to the action of water vapor during the period of calcination.

2. The method of defiuorinating a phosphate rock containing silica in excess of 4% and tluorine in excess of 1%, which comprises subjecting the rock to a silica-removing treatment and reducing its silica content to less than 4%, calcining at a temperature between about 2700" F. and about 3000 F. without substantial fusion a charge containing less than 4% of silica and composed predominantly of the treated rock ground to a neness of at least 65% through 200 mesh, and subjecting all parts of the charge to the action of water vapor during the period of calcination.

3. The method of deuorinating a phosphate rock containing silica in excess of 4% and fluorine in excess of 1%, which comprises subjecting the rock to a silica-removing treatment and reducing its silica content to less than 4%, calcining without substantial fusion a charge in the form of porous nodules containing less than 4% of silica and composed predominantly of the treated rock, the calcining temperature being at least 2700 F. but not so high that substantial fusion of the charge takes place, and subjecting all parts of the charge to the action of water vapor during the period of calcination.,

4. The method of, defluorinating a phosphate rock containing silica in excess of 3% and iiuorine in excess of 1%, which comprises subjecting the rock to a silica-removing treatment and reducing its silica content to less than 3%, calcining without substantial fusion a charge containing less than 3% silica and composed predominantly of the treated rock at a temperature between about 2700".F. and about 3000 F., and subjecting all parts of the charge to the action of water vapor during the period of calcination.

ERNEST J. MAUST.

CLINTON A. HOLLINGSWORTH.

REFERENCES CITED The following references are of record in the ille of this patent:

UNI/TED STATES PATENTS Number Name l Date 1,032,763 Newberry et al July 16, 1912 1,396,149 Soper Nov. 8, 1921 5 1,902,832 Caldwell Mar. 28,1933 2,093,176 Tromel Sept. 14, 1937 2,162,609 Dawe June 13, 1939 2,189,248 Luscher Feb. 6, 1940 2,279,033 Dolbear Apr. 7, 1942 2,283,174 Bates May 19, 1942 -FOREIGN PATENTS Number Country Date 418,788 Great Britain Oct. 31, 1934 424,045` Great Britain .4. Feb. 13, 1935 OTHER REFERENCES Marshall et al.: Industrial and Engineering Chemistry. v01. 27, No. 2 (1935), pages 205-209. 

